Yes you will be able to use properties like background-gradient and border-radius in your SASS and have them work in IE6. We plan on releasing a blog post with further details explaining this, but in short, we have created tools that automatically slice up the images required to get everything looking the same across all browsers. Including IE6. Further details will be given soon.

P.S. If you look at tabs and buttons in IE you can see an example of the sprites being generated by this tool. Note that these tabs and buttons are all styled using CSS3 properties background-gradient and border-radius. Cool uh?